The present invention relates generally to vision correction systems. In one embodiment, the present invention relates to a simplified optical feedback system which can be integrated into existing laser eye surgery systems to provide feedback regarding the progress of the changes in refractive characteristics of the eye, optionally allowing real-time measurements of the rate of change in quality of the ocular optical system of the eye during vision correction surgery.
Known laser eye procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye to alter the refractive characteristics of the eye. The laser removes a selected portion of the corneal tissue, often to correct refractive errors of the eye. Ultraviolet laser ablation results in photodecomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photochemically, directly breaking the intermolecular bonds.
Laser ablation procedures can remove the targeted stroma of the cornea to change the cornea""s contour for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and the like. Control over the distribution of ablation energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, fixed and moveable apertures, controlled scanning systems, eye movement tracking mechanisms, and the like. In known systems, the laser beam often comprises a series of discrete pulses of laser light energy, with the total shape and amount of tissue removed being determined by the shape, size, location, and/or number of a pattern of laser energy pulses impinging on the cornea. A variety of algorithms may be used to calculate the pattern of laser pulses used to reshape the cornea so as to correct a refractive error of the eye.
Although known algorithms have generally been successful in calculating the pattern of laser energy to apply to correct standard vision errors, current vision correction systems would be further improved if they could monitor the changes actually taking place during a photorefractive procedure. Known ablation algorithms often assume a uniform ablation rate, so that each pulse of laser energy is expected to remove a uniform depth of corneal tissue. Although this is often a valid approximation, ablation depths may vary significantly with changes in environmental conditions, such as at different humidities or the like. Ablation depths may also vary locally, such as with the phenomenon called xe2x80x9ccentral islands,xe2x80x9d a slightly reduced central ablation depth sometimes experienced within a large area ablation. As a result of ablation depth inconsistencies, touch-up procedures are sometimes performed following laser surgery after the eye has healed in order to further reshape the cornea and provide the desired vision performance. Furthermore, as healing can take several months, these touch-up surgeries can create a substantial inconvenience for a patient. To avoid this delay, laser surgery systems would benefit greatly from having some type of concurrent feedback.
Treatment of still further refractive errors of the eye have also been proposed, including treatment of irregular corneas and the like. Hartmann-Shack wavefront sensor topography devices are now being developed to accurately measure the optical characteristics of the eye. Theoretically, custom ablation patterns derived from such measurement systems may allow correction of small irregular errors with sufficient accuracy to reliably provide visual acuities of better than 20/20. Unfortunately, the wavefront sensors proposed to date have been quite bulky, so that it may be difficult and/or impossible to incorporate these measurement devices into the existing laser surgery systems now in use. While it may be possible to include an alternative off-axis cornea measurement system in known treatment devices, the accuracy of such off-axis systems may not be as good as desired, particularly for treatment of minor irregular errors of the eye so as to maximize visual acuity. Hence, alternative techniques are needed to provide feedback on the actual progress of an ablation. Such feedback techniques might provide substantial benefits over conventional procedures, where a patient generally waits for the epithelium or flap covering the ablated stromal surface to heal before the eye is further evaluated and before xe2x80x9ctouch upxe2x80x9d surgery can be performed to further reshape the cornea.
In light of the above, it would be desirable to provide improved ophthalmological systems, devices, and methods. It would be particularly desirable to provide enhanced techniques for verifying the success of a laser eye surgery procedure. It would further be desirable if these devices could be easily integrated into existing laser eye surgery systems, as well as in newly developed surgery systems. At least some of these objectives will be met by the system and method of the present invention described hereinafter and in the claims.
The present invention provides improved laser eye surgery devices, systems, and methods. More particularly, the present invention provides devices, systems and methods which can provide measurements of the refractive error in the eye before, during, and/or after vision correction surgery, often while the patient is positioned for laser treatment and aligned with the laser delivery system. The present invention allows adjustments to be made during the vision correction operation, without having to wait for post-surgery analysis regarding the success of the surgery. This is particularly useful when the patient""s eye has unusual characteristics which may not have been accounted for and/or if there are unanticipated difficulties in the operation, such as an error in measuring the original patient prescription, human operator error, variations in humidity, or the like. By taking advantage of a relatively simple system for determining the optical properties of a patients eye, with many of the system components already being included on known laser treatment workstations, the present invention may be used to provide vision better than 20/20.
In a first aspect, the invention provides an eye treatment system for performing vision correction on an eye. The eye has retina and ocular optics including a cornea. The system comprises projection optics arranged to project a reference image through the ocular optics and onto the retina when the eye is positioned for treatment. Imaging optics are oriented to acquire an evaluation image from the retina through the ocular optics. The evaluation image is defined by the reference image as projected through the ocular optics and imaged through the ocular optics. An energy transmitting element is positioned relative to the imaging optics to transmit treatment energy toward the cornea for altering the ocular optics.
In many embodiments, at least a portion of the portion of the projection optics and/or the imaging optics will be coaxially aligned with the treatment energy. Typically, the energy transmitting element comprises a laser, with the energy comprising a corneal ablation laser beam directed along a beam path. Beam splitters can be provided to separate the beam path from an imaging path of the imaging optics, a projection path of the projection optics, and the like, with the projection and imaging paths each having at least a portion coaxially aligned with the beam path of the laser beam.
Advantageously, the imaging optics may comprise a microscope such as the microscopes often included in laser eye surgery systems to image the cornea for optically directing a resculpting procedure. Such corneal imaging microscopes may be modified to allow imaging of the evaluation image from the retina by including additional and/or selectable lenses along the imaging path, by providing sufficient travel of movement of the microscope body, or the like. Typically, an imaging beam splitter will separate a microscope optical path from the imaging path before the image reaches the eyepiece of the microscope.
In many embodiments, an image capture device such as a Charge Couple Device (CCD) will be optically coupled to the imaging system to generate signals in response to the analysis image. An image analyzer will often be coupled to the image capture device, with the image analyzer generally determining an imaging quality of the ocular optics. The image analyzer will often be coupled to the energy transmitting element to define a corneal treatment feedback path. In many embodiments, a reference object will define the reference image and the image analyzer will compare an image of the reference object to determine the imaging quality. The analyzer will often calculate the imaging quality using a modulation transfer function, the analyzer ideally calculating a rate of change of the imaging quality (such as a slope of the image quality relative to the treatment energy directed to the cornea), so that the system can terminate the treatment energy at or below a predetermined (often low) rate of change of the imaging quality.
In some embodiments, the projection optics will include at least one moveable element to adjust a focal distance between the eye treatment system and the projected reference. Optionally, the eye surgery system may include a patient target fixation system which makes use of at least a portion of the projection optics. The target system may be capable of directing a fixation target toward the eye for viewing by the eye so as to help the patient maintain axial alignment between the eye and the treatment energy. Optionally, the at least one moveable element may be adjusted during treatment of the eye while monitoring the analysis image so as to help determine the change in refractive properties actually effected by the treatments. In a simple embodiment, the system operator may vary the projection focal distance between treatments so as to estimate one or more optical characteristic (such as quality, power, or the like) and/or one or more rate of change of an optical characteristic effected by a partial treatment of the eye.
In another aspect, the invention provides a method for performing vision correction on an eye. The method comprises aligning the eye relative to a treatment axis of a treatment system. A refractive characteristic of the eye is changed by directing a laser beam along the treatment axis and onto a cornea of the eye. An image is projected onto a retina of the aligned cornea through an ocular optic system, the ocular optic system including the cornea. The projected image from the retina is imaged through the ocular optic system, and the laser beam is controlled at least in part in response to the imaging step.
Preferably, an optical imaging quality of the ocular optical system will be determined based on an analysis image defined by the imaging step. The imaging quality will often be determined by comparing the analysis image with a reference image, typically using an optical transfer function or the like. In the exemplary embodiment, a rate of change of the imaging quality will be determined, which can thereby indicate combined distortions associated with a first pass of the projected image through the ocular optical system and onto the retina, as well as a second pass of the image through the ocular optical system from the retina to the imaging system. The optical imaging quality and/or its rate of change may be calculated by a processor, or may simply be monitored by a system operator. Regardless, this can provide a feedback indication of the progress of the actual ablation procedure during laser eye surgery.
A further understanding of the nature and advantages of the invention will become apparent by reference to the remaining portions of the specification and drawings.